The present invention relates to a lateral semiconductor device, particularly, a lateral insulated gate bipolar transistor (to be simply referred to as an IGBT hereinafter), and to a vertical semiconductor device, particularly, a vertical IGBT.
An IGBT as a insulated gate type high-voltage semiconductor device is a voltage-controlled semiconductor device. Since this facilitates the formation of a gate circuit, an IGBT is widely used in the field of power electronics such as inverters and switching power supplies. In particular, an IGBT is a power device having both high-speed switching characteristics of a MOSFET and high-output characteristics of a bipolar transistor. Also, a lateral IGBT which is advantageous in high integration is often used as an output device of a power IC. A power IC including a plurality of output devices is in many times fabricated using an SOI (Semiconductor On Insulator) substrate which is advantageous in dielectric isolation.
A lateral IGBT of this type related to the present invention will be explained below with reference to FIGS. 24 and 25. FIG. 24 is a plan view of the IGBT. FIG. 25 is a sectional view taken along a line A-A′ in FIG. 24.
An SOI substrate 1101 has a support substrate 1102, a buried oxide film 1103, and an n−-type base layer 1104. An n-type buffer layer 1105 is formed in the surface of the n−-type layer 1104 by selective diffusion. This n-type buffer layer 1105 has a stripe shape whose two end portions protrude outward into the shape of an arc. A p-type drain layer 1106 is formed in the surface of the n-type buffer layer 1105 by selective diffusion. This p-type drain layer 1106 has the same shape as the n-type buffer layer 1105.
In the surface of the n−-type base layer 1104, a p-type base layer 1107 is formed by selective diffusion so as to surround the n-type buffer layer 1105. The inner circumferential surface of this p-type base layer 1107 has the same shape as the n-type buffer layer 1105. Striped n+-type source layer 1108 are formed in portions of the p-type base layer 1107 by selective diffusion on the two sides of the p-type drain layer 1106. These n+-type source layers 1108 have substantially the same length as the straight portion of the p-type drain layer 1106.
On the p-type base layer 1107 sandwiched between the n−-type base layer 1104 and the n+-type source layers 1108, a gate electrode 1110 is formed via a gate insulating film 1109. This gate electrode 1110 is formed into an annular structure so as to surround the n-type buffer layer 1105. The inner circumferential surface of the gate electrode 1110 has the same shape as the outer circumferential surface of the n-type buffer layer 1105. In addition, a gate line 1113 for extracting the gate electrode to the outside is formed in a portion of the gate electrode.
An insulating film 1111 is formed on the exposed surfaces of the gate electrode 1110 and the n−-type base layer 1104. A drain line 1112 and a source line 1114 are formed on this insulating film 1111. Contact holes 1115 are formed in predetermined positions of the insulating film 1111. Through these contact holes 1115, the drain line 1112 is in ohmic contact with the p-type drain layer 1106, and the source line 1114 is in ohmic contact with the p-type base layer 1107 and the n+-type source layer 1108.
To obtain a high breakdown voltage in this lateral IGBT, a curvature R of the arc at the two end portions of the n-type buffer layer 1105 must be increased to some extent. To this end, a width Lb of the n-type buffer layer 1105 must be increased. If this width Lb of the n-type buffer layer 1105 is increased, the width of the p-type drain layer 1106 also increases, and this inevitably increases the area of the p-type drain layer 1106.
However, it is found by the experiments conducted by the present inventors that when the area of the p-type drain layer 1106 is increased by increasing the width Lb of the n-type buffer layer 1105, the ON voltage of the IGBT rises. FIG. 26 is a graph showing the relationship between the area of the p-type drain layer and the ON voltage of the IGBT. As shown in FIG. 26, this IGBT has the problem that when the width Lb of the n-type buffer layer 1105 is increased in order to obtain a high breakdown voltage, the area of the p-type drain layer 1106 increases, and this raises the ON voltage.
A vertical IGBT relevant to the present invention will be described next. FIG. 27 is a longitudinal sectional view showing this vertical IGBT.
This IGBT includes a drain electrode 1201, a p-type drain layer 1202, an n-type buffer layer 1203, an n−-type base layer 1204, a p-type base layer 1205, an n+-type source layer 1206, a gate insulating film 1207, a gate electrode 1208, and a source electrode 1209.
In this structure, when a voltage which is positive with respect to the source electrode 1209 is applied to the gate electrode 1208 while a voltage which is positive with respect to the source electrode 1209 is applied to the drain electrode 1201, the n+-type source layer 1206 is electrically connected to the n−-type base layer 1204 via a channel formed on the surface of the p-type base layer 1205 below the gate electrode 1208, so electrons are injected into the n−-type base layer 1204. Also, holes in an amount corresponding to the injected electrons are injected from the p-type drain layer 1202 into the n−-type base layer 1204.
This lowers the resistance of the high-resistance n−-type base layer 1204 by conductivity modulation. Accordingly, the ON voltage can be made lower than that of a MOSFET having the same forward-blocking characteristics.
To turn off this IGBT, the application of the positive voltage to the gate electrode 1208 need only be stopped. Consequently, the injection of electrons into the n−-type base layer 1204 stops, and the injection of holes stops accordingly. However, electrons and holes remaining in the n−-type base layer 1204 keep flowing for a while as a recombination current which depends upon the lifetime of the n−-type base layer 1204, and a drift current resulting from the spread of a depletion layer caused by the voltage rise.
To reduce the loss upon turning-off of the IGBT while the ON voltage is kept low, therefore, as shown in FIG. 28, it is necessary to increase the carrier amount in the source electrode 1209 and reduce the carrier amount in the drain electrode 1201. This is so because the depletion layer extends from the source and carriers in the drain remain to the last.
As a method of reducing the carrier amount in the drain, a method using the lightly doped p-type drain layer 1202 is proposed in the following reference.
J. Fugger et al., “Optimizing the vertical IGBT structure—The NPT concept as the most economic and electrically ideal solution for a 1200V IGBT”, Proceedings of the 8th ISPSD, pp. 169-172, 1996.
In this method, it is necessary to form the n-type buffer layer 1203 at a minimum necessary concentration in order to hold the forward-blocking voltage, and to form the p-type drain layer 1202 at a low concentration in order to suppress the injection of holes.
The p-type drain layer 1202 is formed by ion implantation of boron and diffusion of the boron by high-temperature annealing. However, surface recession caused by the diffusion lowers the surface concentration of the boron, so no ohmic contact to the drain electrode 1201 can be formed, and injection of holes hardly occurs. Also, since the implantation dose of the boron is small, the device characteristics largely vary if the dose varies even slightly. This results in a small process margin.
Another problem of the vertical semiconductor device will be described below. Although an IGBT is a low-loss semiconductor device, attempts for decreasing the substrate thickness are recently being made to further reduce the loss. For example, the substrate thickness of an IGBT having a withstand voltage of 600 V is decreased to 50 μm.
However, when the n−-type base layer is thinned by thinning the substrate, the substrate cracks in the device fabrication process.
As described above, the lateral semiconductor device related to the present invention has the problem that when the width Lb of the n-type buffer layer 1105 is increased in order to obtain a high breakdown voltage, the area of the p-type drain layer 1106 increases, and this raises the ON voltage.
Also, in the vertical semiconductor device described above, a very-lightly-doped p-type collector layer 1202 must be formed in order to obtain a good trade-off relationship between the ON voltage and the turn-off loss. However, in ion implantation and diffusion by high-temperature annealing as the formation method of this layer, the surface concentration is difficult to control. This causes variations in the device characteristics.
Furthermore, when the n-type base layer is thinned by thinning the substrate to thereby reduce the loss, the substrate cracks during the device fabrication process.